In recent SEMs, electrons emitted from a sample by electron beam irradiation are efficiently guided into an objective lens, and then its electron energy is discriminated so that electrons within a specific energy range may be captured. This electron detection method is called an in-lens detector because of the structure where the detector exists in the lens. By using this detector, selective imaging can be done from the information contained in the secondary electrons, the backscattered electrons or the like, allowing visualization of the information on, e.g., the composition, variation of the surface potential, which have never been visible. This has dramatically expanded the application field of SEM (Non Patent Literature 1).
On the other hand, as another type electron detector, there is an electron detector (out-lens detector, also referred to as an Everhart-Thornley type) which is installed on the wall of a sample chamber between an objective lens and a sample. The out-lens detector has been utilized since the early stage of SEM development, and the out-lens detector is adopted in many general-purpose SEMs. The out-lens detector images the secondary electrons by applying an electric field on the electrode in front there of the detector for collecting the electrons emitted from the sample.
Though it is generally considered that the out-lens detector effectively captures high energy electrons, the high energy electrons emitted from the sample often travel straight and impinge on a wall surface of the sample chamber, resulting in being scattered therefrom or emitting other electrons (these electron are referred to as SE3 electrons), which electrons in turn reach the out-lens detector. For this reason, the out-lens detector cannot recognize the origin of the secondary electrons that contribute to the image, and thus, the out-lens detector has almost always been used for morphological observation.
In addition, since the SE3 electrons has lost the energy on emission from the sample, their original information has been lost, and thus the captured SE3 electrons result in noise. It is noted that, although various pieces of equipment are installed inside the sample chamber of electron microscope, all of those except for the out-lens detector and the sample installed within the sample chamber that do not directly relate to the present invention are regarded as portions of the wall of the sample chamber in the present application, as far as the impingement of secondary electrons and the emission of SE3 are concerned.
As described above, the in-lens detector added with the energy discrimination function has been developed, and the application range thereof has been greatly widened. However, typical out-lens detectors have no energy discrimination function. For this reason, the importance of the out-lens detector is lowered. In addition, examples of the in-lens detectors in cited techniques are disclosed in Patent Literatures 1 and 2.
Furthermore, in a case where a sample of an insulator is observed with an accelerating voltage of about 10 kV, the sample is often negatively charged. However, in the case of observing such a negatively charged sample, since electrons emitted from the sample are affected by charging to be accelerated, trajectories of the secondary electrons are greatly deviated from the direction toward the detector, and thus, a proportion of the electrons directly captured by the out-lens detector is decreased, so that most of the electrons trapped in the out-lens detector become SE3 electrons. This situation is illustrated in FIGS. 1(a) and 1(b). FIG. 1(a) is a diagram illustrating trajectories drawn by secondary electrons having energy of 10 eV emitted from a sample in a case where the sample is charged (−50 V) at slightly lower left from the center in a region having a basically rectangular cross section installed inside a conductor, and FIG. 1(b) is a diagram illustrating trajectories drawn by secondary electrons emitted from a sample in a case where the sample is not charged.
Herein, the potential of an out-lens detector (the protruding distance is the distance from the position of the right wall surface to the center of the slanted distal end of the detector) protruding 75 mm in the left direction from the vicinity of the upper end of the right wall surface is set to +150 V. In addition, the shading of the lines indicating the trajectories represents the emitting direction (angle) of the secondary electrons from the sample. In addition, with respect to the secondary electrons emitted from the same point on the sample in the same direction, the influence from the electric field varies with the energy of the secondary electrons, and the secondary electrons have different trajectories accordingly. In addition, In FIGS. 1(a) and 1(b), around the sample and the detector, lines indicating equipotential surfaces formed around the detector and the sample by such potentials are drawn by light colors (around the detector) and broken lines (around the sample). Herein, since the sample of FIG. 1(b) is not charged, the potential of the sample is the same as that of the surrounding wall surface. Therefore, since the gradient of the potential in the vicinity of the sample is small, broken lines representing an equipotential surface is not drawn around the sample in FIG. 1(b).
As clarified from FIGS. 1(a) and 1(b), the trajectories of the secondary electrons emitted from the charged sample are greatly deviated from the direction toward the detector. Therefore, in the case of observing a sample of an insulator, there is a problem in that the signal intensity is greatly lowered.